Applicant is the inventor of the “quick change” bit holder/bit holder block combination that enables a bit assembly to have its bit holder retained in the bit holder block without the use of threaded nuts or spring clips holding the bit holder shank in the bit holder block bore. This invention is shown and discussed in applicant's U.S. Pat. Nos. 6,371,567, and 6,585,326 and RE44,690.
After the creation of the retainerless bit holder shank, applicant realized that the combination of a like tapered shank together with its corresponding bit holder block bore, when retained by an excessive interference fit made possible by an axial slot in the hollow shank, resulted in the greatest interference force being positioned adjacent the top of the terminus of the slotted distal tapered portion of the bit holder shank. The increased interference between the bit holder block bore and bit holder shank caused the minute radial and circumferential collapsing of the bit holder shank adjacent the axial oriented slot. The collapse became more prominent toward the distal end of the shank resulting in somewhat less radial and circumferential force being applied toward that distal end by the bit holder block bore. After the time of the conception of the invention found in the ‘567 and ‘326 patents, the positioning of greatest holding force between the bit holder shank and bitholder block bore adjacent the top of slot was utilized in an effort to ease the insertion of the bit holder in the bit holder block bore. That greatest portion of force to insert the bit holder shank was positioned at the last ¼ to ¾ inch of the insertion of the shank of the bit holder in the bit holder block bore. Additional holding force was added adjacent the top of the shank with a standard annular interference fit with the corresponding top portion of the bit holder block bore. These two greatest holding force positions, when added to the holding force positioned adjacent the remainder of the distal tapered portion of the shank resulted in the 3,000 to 15,000 lbs. necessary to insert and retain a lubricated shank in the bit holder block bore. These axial insertion forces are derived when preferably a molybdenum disulfide type lubricant is smeared on the shank of the holder. At least double the axial removal force is required when no lubricant is used.
In road milling machinery, bit assemblies are generally positioned around the outside of a cylindrical drum that is dimensioned to rotatably fit within the confines of the underside of a road milling machine. In an effort to create the smoothest road milling, bit assemblies have been mounted in staggered positions in spiral or chevron form on the drum to decrease the axial dimensions between adjacent cutting tips of bit assemblies. As a result of this spiral or chevron orientation and the positioning of many bit assemblies on a current milling drum, the spacing behind each bit assembly has been reduced to the point where access to the rear of each bit assembly is severely limited. This rear access is necessary in order to drive out shanks of broken bits, shanks of broken bit holders, and conical bits. The shorter bit holder shank and the shorter bit holder block bore now provide this access.
When increasing the rear access to the distal end of the shank by shortening the axial shank length and by shortening the axial length of the bit holder block bore, the holding force previously there is reduced. That shank to bit holder block bore retention force must be re-established.
A need has developed to provide an improved bit holder/bit holder block bore assembly wherein the holding force between a bit holder shank and a bit holder block bore may be positioned axially along the bit holder shank as desired, rather than as previously positioned on the aforementioned portions of the shank. Additionally, increased access to the rear of the bit holder block is desirable for easing the ability to drive the bit holder shank and bit shanks out of its bit holder block from the rear thereof.